This invention relates in general to coking and in particular to a new and useful method and apparatus for reducing fine coal emission during the time that predried and preheated coal is charged into coke ovens.
Installations for drying and preheating coal in flash driers prior to charging into coke ovens are known in the prior art.
To heat the coal fed into the preheating stage uniformly and rapidly, the coal must be relatively finely ground. Due to abrasion during this additional grinding operation and during the heating, a relatively high proportion of very fine coal is obtained in the coal mixture. Upon charging the preheated coal into the oven chambers, a portion of the very fine coal (up to 5% of the charge) passes with the charging gas into the collector main and then into the gas condensate-tar-water mixture (known in the art as "carryover") and makes the tar unusable for further processing, because of the high dust load.
Another difficulty connected with the charging of hot coal is that this freely flying dust escapes from the system through the smallest of leaks in the transfer areas and becomes a considerable environmental nuisance.
Numerous designs have already been provided in the past to prevent the finest coal dust from penetrating to the atmosphere and into the gas condensation system.
According to a prior art method (German OS No. 2,514,859), coal is wetted with 0.5 to 3% by weight of bituminous coal tar and then introduced into the ovens by pouring, with the tar containing about 3 to 7% of water and up to 1 weight percent of wetting agents. It is further provided that only the fine portion of the preheated coal is wetted with the tar and then blended with the main amount of preheated coal.
In a similar way, German AS No. 2,514,007 provides that 0.5 to 1 parts per mill of an aqueous 30 to 70% solution of an adhesive is admixed.
However, experience has shown that due to this so called admixture of additives, the bulk density of the coal as well as the quantity of the produced coke are reduced.It has further been found that with this method, emission at leaking spots during the transportation and charging of hot coal cannot be prevented.
Also, it is fairly difficult to admix the additives uniformly and, especially, to bind the fine dust to the coarser pieces.
In prior art, flash driers and coal preheaters, the heated coal is separated from the heating gases in a two-stage material separating system (usually cyclones) where the greatest part of about 95% of the total amount is separated in the first stage, and the remaining finer fractions of 4 to 4.5% in the second stage. Still before the heated gases leave the heater, the very finest dust is separated in a following electrostatic filter or wet scrubber.
The fundamental reason for this two-stage separation is that in the first stage, predominantly coarser fractions are separated, with the first stage being relatively amply dimensioned, so that the cyclone runs only at low speeds and wear is reduced. The second stage ordinarily comprises a plurality of cyclones connected in tandem or in parallel and having a small diameter, permitting a separation of the finer particles since the rotational speeds are high. Here, in spite of the high speeds, no wear is to be expected because of the small inertia of the fine particles.
Grain size analysis in various installations shows that the coal dust entrained into the gas condensation system does consist primarily of the finest particles, however, also of particles up to about one millimeter.
If now the grain size of the preheated coal is compared with that of the dust separated in the second stage, the following is found:
A complete analysis of the preheated coal reveals about 20% of a grain smaller than 0.1 millimeter.
An analysis of the dust separated in the second stage shows 100% of a particle size smaller than 0.05 millimeter, that is about 5% of the total amount.
In the first stage, despite the large size, already about 15% of the fine coal with a grain size smaller than 0.1 mm, and even the finest particles smaller than 20 microns are separated from the gas stream along with the coarse coal. This means that in the prior art systems it would be necessary to admix an additive, in accordance with German AS No. 2,514,007 or German OS No. 2,415,859, not only to the fine coal but also to the coarse coal, in order to satisfactorily bind the entire fine coal to the coarse fraction. This, however, is very expensive.